Such proximity sensors are known in a variety of configurations, e.g. as disclosed in our U.S. Pat. No. 4,193,023 and earlier patents referred to therein. In all these cases the output of the proximity sensor is fed to a control circuit which either energizes or de-energizes a load when the object comes within a predetermined distance from the observation point or occupies a certain position relative thereto; the load could be a simple indicator (visual or aural, for example) and/or a switch starting or stopping an associated mechanism. Devices of this character can also be used as quantitative indicators of the distance of a nearby object from a reference point.
A distinction can be made between two types of contactless proximity sensors, namely those which respond inductively and those which respond capacitively to the approach of an external object.
A conventional capacitive sensor comprises and oscillator which is inoperative until an external capacitance including the object completes a tank circuit therefor. Inductive sensors, on the other hand, have free-running oscillators with tank circuits whose inductive branches have their impedances modified by the approaching object. Generally, sensors of the latter type have fewer problems than the capacitive ones, on account of the continuous oscillation generation. Inductive sensors, moreover, are found to have a more favorable ratio of induced damping to inherent damping. Still, certain objects--e.g. those of low conductivity--are more readily detectable by capacitive sensors.